1. Field of the Invention
The present invention relates to an image forming apparatus wherein an image is formed on an image bearing member and then is transferred onto a recording material to obtain a copy image. For example, the present invention is particularly applicable to color copying machines of electrophotographic type and color printers wherein a plurality of toner images having different colors are successively formed on an electrophotographic photosensitive body and the plural color toner images are successively transferred onto a single sheet in a superimposed fashion to obtain a color image.
2. Related Background Art
In the past, there have been proposed various color image forming apparatuses wherein a plurality of toner images having different colors are successively formed on an image bearing member and such plural color toner images are successively transferred onto a single recording material in a superimposed fashion to obtain a color image. Among them, copying machines of multicolor electrophotographic type are used most popularly.
An example of such a color electrophotographic copying machine will now be briefly explained with reference to FIG. 5. As shown in FIG. 5, the color electrophotographic copying machine includes a photosensitive drum (image bearing member) 1 around which an exposure lamp 21, a first charger 2, a light source (not shown) and a polygon mirror 17 are arranged. A laser beam emitted from the light source is scanned by rotation of the polygon mirror, and scanned light deflected by a reflection mirror is focused on the generatrix of the photosensitive drum 1 through an f.theta. lens to expose the photosensitive drum, thereby forming latent images for various colors corresponding to an image signal on the photosensitive drum 1 successively.
The latent images for various colors formed on the photosensitive drum 1 are successively developed by a rotatable developing means 3 at a developing station where the photosensitive drum 1 is opposed to the developing means, thereby visualizing the latent images as toner images. The rotatable developing means 3 includes a rotatable table on which a yellow developing device 3a, a magenta developing device 3b, a cyan developing device 3c and a black developing device 3d are mounted, and yellow color developer, magenta color developer, cyan color developer and black color developer are supplied from respective developer supply sources to the respective developing devices 3a to 3d by a predetermined amount. The various color toner images formed on the photosensitive drum 1 are successively transferred onto a recording material 6 supported by a transfer drum (recording material bearing member) 8 at a transfer station where the photosensitive drum 1 is opposed to the transfer drum. The recording material 6 is sent from a recording material cassette 60 to the transfer drum 8 via a pair of resist rollers 13 and then is born on the transfer drum.
As shown in FIG. 6, the transfer drum 6 has a cylindrical frame comprised of a pair of rings 8b interconnected by a connection member (frame connection member) 8c, and a dielectric sheet 8a as a recording material bearing sheet covering a peripheral opening area of the cylindrical frame. The dielectric sheet 8a may be formed from a film made of polyethylene terephthalate (PET), polyvinylidene fluoride or polyurethane resin.
Next, the entire operation of the above-mentioned color image forming apparatus will be explained. A photosensitive layer on the photosensitive drum 1 is uniformly charged by the first charger 2, and the charged drum is firstly exposed by laser light modulated by a yellow image signal corresponding to a yellow color in an image on an original, thereby forming an electrostatic latent image corresponding to a yellow image on the photosensitive drum 1. The latent image is brought to the developing station by rotation of the photosensitive drum 1, where the latent image is visualized as a yellow toner image by the yellow developing device 3a previously positioned at the developing station.
On the other hand, the recording material 6 from the recording material cassette 60 is sent to the transfer drum 8 via a sheet supply roller, regist rollers 13 and a sheet supply guide and is moved along the transfer drum 8. In this case, an absorption roller of an absorption charge means 22 (which also includes an absorption charger) is urged against the dielectric sheet 8a of the transfer drum 8, and at the same time, the recording material is charged by the absorption charger so that the recording material 6 is electrostatically absorbed onto the dielectric sheet 8a.
The recording material 6 born on the transfer drum 8 is brought to the transfer station where the photosensitive drum 1 is opposed to the transfer drum, by the rotation of the transfer drum. In the transfer station, a transfer electric field is generated by a transfer charge means 4 (comprising a conductive transfer charge brush contacted with a back surface of the dielectric sheet 8a at the transfer station in this example), so that the yellow toner image formed on the photosensitive drum 1 is transferred onto the recording material 6. The transfer drum 8 continues to rotate so that the recording material 6 to which the yellow toner image was transferred is sent to the transfer station again in order to transfer a next magenta toner image onto the same recording material.
On the other hand, after the transferring, the photosensitive drum 1 is cleaned by a cleaning member 5. Then, the photosensitive drum is uniformly charged by the first charger 2 again and then is exposed by laser light modulated by a magenta image signal to form an electrostatic latent image corresponding to a magenta image. The latent image is developed by the magenta developing device 3b positioned at the developing station, thereby forming a magenta toner image. The magenta toner image is then transferred onto the recording material at the transfer station in a superimposed relation to the yellow toner image.
By repeating the same operations regarding the cyan color and the black color, the yellow, magenta, cyan and black toner images are superimposed onto the recording material 6, thereby obtaining a color image. After the four color toner images were transferred, the recording material 6 is separated from the transfer drum 8 by a separation pawl and then is sent to a fixing device 7 by a convey belt.
The fixing device 7 comprises a fixing roller 71, a pressure roller 72, heat-resistive cleaning members 73, 74 for cleaning these rollers, and heaters 75, 76 for heating these rollers. A coating roller 77 is contacted with the fixing roller 71 so that mold releasing oil reserved an oil reservoir 78 is coated on an outer peripheral surface of the fixing roller 71 via the coating roller. A thermistor 79 for controlling a fixing temperature is contacted with an outer peripheral surface of the pressure roller 72. The recording material 6 sent to the fixing device 7 is heated and pressurized between the fixing roller 71 and the pressure roller 72 heated by the heaters, thereby fusing and mixing the toner images to fix the color image onto the recording material as a permanent image. After the fixing operation, the recording material is discharged out of the apparatus.
After the electricity on the transfer drum is removed by outer and inner electricity removal chargers 14, 15 to remove the electrostatic absorbing force, the developer remaining on the transfer drum 8 is scraped by a rotating fur brush 16. The cleaning means for removing the developer from the transfer drum 8 may be a blade or a non-woven fabric web, other than the fur brush. Of course, these elements may be used in combination.
As mentioned above, in the color image forming apparatus having the above-mentioned arrangement, the toner images are transferred onto the same recording material 6 by four times. By the way, in the past, in order to generate the transfer electric field, a high voltage (as a transfer voltage) from a high voltage power source was applied to the transfer charge means under constant current control or constant voltage control. However, for example, in the constant current control, a maximum voltage value (i.e. a maximum output voltage of the high voltage power source) applicable to the transfer charge means was constant for four colors.
Thus, among the plurality of transferring operations, in the transferring operation for the first color (yellow), there arose a problem that excessive transfer electric field is generated at the transfer station. Further, FIG. 8 shows a typical example that the voltage is applied to the transfer charge means 4 under the constant current control. In FIG. 8, a terminal 201 is connected to the transfer charge means 4. In this case, by feeding-back a current detection portion 202 to a primary side of a transformer, a switching power source for effecting the constant current control is obtained (although the present invention is not limited to such a control method, but explanation will be continued with reference to this example).
However, in the above example, since the maximum electric power (maximum voltage under the constant current control, and maximum current under the constant voltage control) to be applied to the transfer charge means is determined without differentiating between the case where an imaging portion (onto which the image can be formed in response to any image information) of the image bearing member is positioned at the transfer station and the case where a non-imaging portion (onto which the image cannot be formed in response to any image information) of the image bearing member is positioned at the transfer station, the excessive charging occurs in local positions of the non-imaging portion (for example, tip end portion of the recording material, rear end portion of the recording material or connection member 8c of the cylindrical frame in FIG. 6).
That is to say, when the high voltage power source is turned ON/OFF or when a portion of the frame connection member 8c of the transfer drum 8, a portion of the dielectric sheet 8a immediately after that portion of the frame connection member (portion of the dielectric sheet 8a near an upstream end of the frame connection member 8c in a rotating direction of the transfer drum) and a portion of the dielectric sheet 8a immediately after the rear end of the born recording material 6 (portion of the dielectric sheet 8a near an upstream rear end of the recording material 6 in the rotating direction of the transfer drum) passes through the transfer station, if the high voltage is applied to the transfer charge means 4 under the constant current control, voltage fluctuation will occur, thereby applying excessive voltage, i.e. excessive transfer electric field to the transfer station.
The excessive voltage is a voltage greater than voltage generating the transfer current under the normal transferring operation. As an example, FIG. 7 shows change in output voltage of the high voltage power source when the first to four color toner images are transferred onto the recording material while applying the high voltage from the high voltage power source to the transfer charge means under the constant current control. In this case, the dielectric sheet 8a was formed from a PET film including carbon as filler, a transfer charge brush was used as the transfer charge means 4, the maximum output voltage the high voltage power source was 10 kV and the high voltage was applied to the transfer charge means 4 under the constant current control of 10 .mu.A. FIG. 7 will be fully described later.
During the transferring operation, if the abovementioned excessive transfer electric field is locally generated, dielectric breakdown will occur on the applied portion of the dielectric sheet 8a, thereby causing "pin hole" or residual charge remains on the photosensitive drum 1 or the dielectric sheet 8a, thereby distorting the image.